With a recent trend of electronic equipment becoming more miniaturized and higher in component density, a circuit board to mount electronic components has evolved from a conventional single board to a double-sided board and a multilayer board. Accordingly, it is necessary to develop a higher density circuit board capable of integrating as many circuits as possible thereon.
For the manufacture of a high density circuit board, the employment of a laser beam machining method capable of high-speed fine machining is now studied in place of a conventional method of making a hole (through-hole) in a board by drilling that has been widely employed. Also, Japanese Laid-Open Patent No. 6-268345 discloses a method of drilling by a laser beam and a circuit forming board wherein layer connection is made by a means using a conductive paste.
In the technology of fine hole forming and layer connection by using a conductive paste, a slight amount of foreign matter can cause defective connection. In this technology, a board material coated with a film material is drilled. The film material is used as a mask to fill a conductive paste into fine holes. Accordingly, it is necessary to keep clean the entire board including the film material.
However, both of the drilling and laser beam machining methods cause generation of a large volume of chips. And there arises a problem such that the chips stick to the board material and clog the connecting hole (through-hole). Also, there is a fear that even a slight amount of dust in the air may cause clogging of the fine hole. The board material is cleaned before a conductive paste is filled into the hole. After using a film material as a mask for filling the conductive paste, the film material bonded to the board material is removed from the board material. Accordingly, the film material is very low in adhesion to the board material, and the film material is liable to come off the board material.
FIG. 8(a) shows a top plan view of a drilling portion in a conventional method of manufacturing a circuit forming board, and FIG. 8(b) shows a cross-sectional view of the drilling portion. As shown in FIGS. 8(a) and 8(b), polyethylene phthalate (PET) sheets 4a, 4b bonded to the portions of especially low in adhesion may sometimes peel off the board material 1. When such peeling takes place around a through-hole, conductive paste 31 for connection may spread into peeled portion 33 and, consequently, shorting 32 may take place between adjacent through-holes.
Accordingly, it is required to clean the board material without giving excessive stresses to the board material 1 and also without peeling the film materials 4a, 4b. However, it is very difficult to satisfy both of these requirements, resulting in insufficient cleaning of the board material.
As described above, the purpose of drilling the board material 1 is to connect the circuits formed on the surface and back or interlayer of the board with each other. After drilling, a connecting means is formed by metal-plating or conductive paste filling.
The sizes of holes and connecting means in a high density circuit board are very small and fine. Accordingly, even very fine foreign matter can cause serious influences to the reliability of connecting means. Therefore, it is necessary to completely remove such foreign matter. However, in a conventional method, film material 4a, 4b serving as a mask peels off when a great stress is given to the board material 1. And then the film material loses its function as a mask or a conductive paste spreads into the board material. Consequently, there arises a problem such as worsening of the reliability in insulation.
The present invention is intended to provide a method of manufacturing a low-cost highly reliable circuit board and its manufacturing apparatus, realizing a board material having high-quality fine holes.